A recent investigation asked nearly 500 mostly heterosexual individuals about their past experiences cheating on a romantic partner (Selterman, Garcia, Tsapelas, & 2019). Note that having engaged in infidelity was an explicit inclusion criteria for the study, so all participants shared at least one instance of their own infidelity as part of the study. Approximately 95 percent gave examples that included sexual/physical infidelity.

These scholars aren't the first to ask the question of why people cheat (e.g., Barta & Kiene, 2005), but evidence regarding infidelity motives is surprisingly scarce, suggesting the need for empirical inquiry. In their study, Selterman and colleagues (2019) solicited reasons for why people cheated and then focused their analysis on synthesizing the many motives people offered. Eight main motives emerged from their analysis:

Falling out of love. Sometimes (but not always) a deficit in an existing relationship leads people to have extradyadic affairs. Over three quarters (77 percent) of participants indicated that a lack of love for their stable partner, and/or greater love for an extradyadic partner, was a fairly strong reason they cheated.
For variety. Other times, infidelity is not a response to a problem with an existing relationship; rather, it's a reaction to boredom. For many people (74 percent), a desire for variety factors into their cheating behavior. More men explained their infidelity as tied to this reason than women.
Feeling neglected. Similar to feeling a lack of love, some people engage in infidelity as a response to their partner's lack of attention. Participants (70 percent) revealed that feeling neglected was at least moderately tied to their cheating behavior. More women than men recognized this as one of their motives for cheating.
Situational forces. Not every act of infidelity is premeditated and driven by dissatisfaction with a current relationship. Many participants (70 percent) noted that factors of the situation were a key reason they cheated. Maybe they were drinking or in some other way thrown into an opportunity they didn't anticipate. More men recognized this motive as a reason for their cheating than women.
To boost self-esteem. It seems counterintuitive, given that infidelity tends to end with significant personal consequences, but for some people, the act of having an affair can boost their own ego and self-esteem. More than half of participants (57 percent) indicated that enhancing their self-esteem was a motive for their cheating.
Out of anger. This was not the most commonly cited reason, but anger played a role in the affairs of many participants (43 percent). In these cases, cheating was seen as a way to punish a partner or enact revenge.
Not feeling committed. Lacking love and lacking commitment to a current romantic partner are both tied to general feelings of relationship dissatisfaction. They may go hand in hand. In terms of commitment, nearly half (41 percent) of participants indicated that having low levels of commitment to their romantic partners motivated their cheating.
Because of sexual desire. About one-third of participants (32 percent) reported that they were driven to have an affair because of their sexual desire. Maybe in their established relationship, individuals aren't engaging in the frequency of sex, style of sex, or specific sexual behaviors that they want; this can contribute to their reasons to cheat. Men reported this reason more than women (Selterman et al., 2019).

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“Never become so involved with something that it blinds you.
Never forget where you from; someone will remind you.” – DMX, “It’s On